[Cashtab] Do not allow users to list Firma below the bid price

Summary:
Every once in awhile someone lists Firma for 1 XEC instead of 1 dollar (or using the redeem feature)

There is no good reason to do this, so prevent it

Test Plan:
npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18923